The principles of thermal energy storage in phase-change materials are well known. As the materials are heated from an initial phase, such as the solid phase, to a second phase, such as the liquid phase, energy is absorbed. In the temperature range at which the material changes from one phase to another, more energy is required to raise the temperature an additional increment than to raise the temperature by the same increment when the material is not changing phase. This additional energy required at the phase change of the material is called the latent heat of transition.
The heat required for the phase change from liquid to gas is called the latent heat of vaporization. The heat required for the phase change from solid to liquid (and give up in the reverse phase change from liquid to solid) is known as the latent heat of fusion. When a material cools, the energy absorbed at the phase-change point is normally given up. Some materials will cool well below the normal phase change temperature, but still retain the latent heat of transition and remain in the higher temperature phase or state. For example, some materials under some circumstances may be cooled below the temperature at which they normally change from liquid to solid yet remain in the liquid state, thus still retain the latent heat of fusion. A material in this condition is said to be undercooled or supercooled. It is possible to create conditions in an undercooled material that will cause it to change very rapidly from the high-temperature phase to the low-temperature phase, thus giving up the energy stored at the latent heat of transition or fusion rapidly. The energy so released may be put to practical use in many ways.
One example of such a use is as a heel warmer with new borns. Most hospitals require that blood samples be taken from new born infants during the first days after birth. Since one of the biggest body masses of a new born is the heel, blood for tests is generally drawn from the heel area. The problem with the heel as a source of blood is that a new born's blood circulation is poor. If blood circulation in the heel area is not increased before testing, drawing blood for testing may have an adverse affect on the infant and cause complications.
It is well known that heat causes blood flow to increase at the site where the heat is applied. Heat applied to the new born's heel area before blood is drawn for testing will increase blood flow into the heel area and prevent complications from the test. Heat packs have long been used in various forms in the medical and sports fields, and they are particularly useful for warming the heels of new born infants before blood is drawn for various tests.
The art relating to the preparation and use of supercooled salt solutions for heat packs is extensive and there are a large number of different solutions that are effective. Such solutions include, but are not limited to, sodium acetate, sodium thiosulfate, trimethylol ethane hydrates, and the like. In general, the salt and a solvent (most often the solvent is water) are mixed together and then heated to a temperature where all of the salt is dissolved in the solvent. The solution is then allowed to cool slowly to around room temperature.
The triggering of the supercooled solution to activate the crystallization has been accomplished in a number of ways. Often a simple shock to the supercooled salt solution will cause crystallization. However, this type of situation can result in the premature crystallization and release of heat. Thus, the salt crystals in the heat pack will need to be redissolved prior to use.
A variety of devices have been tried for triggering crystallization. For example, U.S. Pat. No. 5,275,156 to Milligan, et al., discloses a trigger device that floats free in the supercooled salt solution which is activated by applying pressure to the device; U.S. Pat. Nos. 4,460,546, 4,899,727 and 4,580,547 to Kapralis, et al., disclose the use of another set of trigger devices which float free in the supercooled salt solution (generally, Kapralis, et al., disclose concave discs which are caused to "snap" in order to activate the heat pack); U.S. Pat. No. 5,056,589 to Hettel, et al., discloses the use of metallic spring mechanism for crystallizing a supercooled salt solution; and U.S. Pat. No. 5,143,048 to Cheney, III, describes a trigger device which comprises a disc or ampule containing crystals of the salt used to form the supercooled salt solution (the disc or ampule is broken and the crystals are exposed to the supercooled salt solution).
There are numerous other devices for activating supercooled salt solutions and each of them has its own advantages. However, the devices of the prior art, in general, have the disadvantage of requiring the user to locate the triggering device prior to activation. The triggering devices usually are floating free in the supercooled salt solution. They must be located in the solution, restrained to a particular position in the heat pack and then activated by some manipulation of the user. It is desirable to have a triggering device which can be used without the necessity of a person having to locate and restrain the triggering device in the supercooled salt solution.
Therefore, it is an object of the present invention to provide a triggering device for a supercooled salt solution type heat pack which provides a trigger that is located in a fixed location relative to, but physically from, the supercooled salt solution.
It is also an object of the present invention to provide such a triggering device which is easy and intuitive to use.
Consideration of the specification, including the several figures to follow, will enable one skilled in the art to determine additional objects and advantages of the invention.